This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Chromophore-assisted laser inactivation (CALI) is a light-mediated technique that offers precise spatiotemporal control of protein inactivation, enabling better understanding of the protein's role in cell function. EGFP has been used effectively as a CALI chromophore, and its cotranslational attachment to the target protein avoids having to use exogenously added labeling reagents. A potential drawback to EGFP-CALI is that the CALI phenotype can be obscured by the endogenous, unlabeled protein that is not susceptible to light inactivation. Performing EGFP-CALI experiments in deficient cells rescued with functional EGFP-fusion proteins permits more complete loss of function to be achieved. We developed a modified lentiviral system for rapid and efficient generation of knockdown cell lines complemented with physiological levels of EGFP-fusion proteins. We demonstrated that CALI of EGFP-CapZbeta increases uncapped actin filaments, resulting in enhanced filament growth and the formation of numerous protrusive structures. We showed that these effects are completely dependent upon knocking down the endogenous protein. To fully interpret these results and also guard against over-interpretation, a quantitative analysis, which build on the Virtual Cell dendritic actin nucleation model, will be developed.